Building a Javascript Runtime Using C

Posted3 months agoActive3 months ago

redbell

84 points

32 comments

devlogs.xyzTechstory

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JavascriptCRuntimeEmbedded Systems

Key topics

Javascript

Runtime

Embedded Systems

The post discusses building a JavaScript runtime using C, but the discussion revolves around whether it's a 'from scratch' implementation and its potential use cases.

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01Story posted
Oct 7, 2025 at 6:18 PM EDT
3 months ago
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02First comment
Oct 11, 2025 at 11:56 AM EDT
4d after posting
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23 comments in 84-96h
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Oct 13, 2025 at 3:13 AM EDT
3 months ago
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Discussion (32 comments)

Showing 32 comments

TheCleric

3 months ago

4 replies

I was a little disappointed that this was “just” a wrapper for JavaScriptCore.

jesse__

3 months ago

1 reply

Yeah, I was expecting a lot more than "I glued some libraries together in C!", especially when the author is claiming 'from scratch'. Seems like a somewhat disingenuous title if you ask me..

jakogut

3 months ago

1 reply

I suppose when your accustomed level of abstraction is interpreted languages like JavaScript, and "the web", "gluing some libraries together in C" is a somewhat novel and interesting endeavor.

jesse__

3 months ago

1 reply

I bit my tongue and decided to hold that jab at JavaScript programmers, but yeah, I think that's exactly what were looking at here

jakogut

3 months ago

I wasn't trying to make any jabs, just an observation that getting outside of your comfort zone can be novel and interesting, even if it's mundane to people that commonly spend lots of time there.

Minor49er

3 months ago

1 reply

Agreed. It contradicts the whole "from scratch" idea. The article even has an engine implementation section where it just calls JavaScriptCore as you mentioned. It's a cool wrapper, but a misleading and disappointing article

trollied

3 months ago

Yup. I clicked on it, based on the title, and expected a long-form article. Not a simple library utilisation post.

curtisblaine

3 months ago

1 reply

To be fair, all commercial non-browser runtimes (node, bun, deno) are "just" wrappers of V8 or JSC. Some more experimental ones are "just" wrappers of QuickJS and other less known engines.

throwawaymaths

3 months ago

iiuc its a runtime because the engine just dispatches one javascript microtask and returns to the runtime with a stack of remaining microtasks

gr4vityWall

3 months ago

Although not intuitive, it's common to call that the 'runtime' in the JS world, while V8 and JSC would be called 'JS engines'.

Deno used similar wording in a tutorial for creating your own JS runtime using Rust and V8 bindings: https://deno.com/blog/roll-your-own-javascript-runtime

IMO the tutorial is still cool nonetheless, it's a fun subject.

yb303

3 months ago

1 reply

This is not "Building from scratch" This is just using.

Spivak

3 months ago

1 reply

Runtime is the glue between JS Engine and OS which is from scratch. Runtime embeds engine and lets engine talk to the outside world.

throwawaymaths

3 months ago

engines only execute one JS microtask at a time, you must run it in something, that's the runtime.

lerp-io

3 months ago

1 reply

unpopular comment : v8 > JavaScriptCore.

dunham

3 months ago

In general, yes, although it's nice to have more than one javascript implementation. And one advantage of JSC is that it implements tail call optimization (per the ES6 spec).

I wrote my own language that targeted javascript. When I made my language self-hosting, I initially used `bun` (based on JSC), so I wouldn't have to implement a tail call transformation myself. It was expedient.

My goal was to run in a browser, so I did eventually add that transformation and found that my compiler was 40% faster running in node (based on v8).

claytongulick

3 months ago

3 replies

For those who would like a true "from scratch" implementation of JavaScript, Fabrice Bellard's QuickJS [1] is clean, readable and approachable. It's a full implementation of modern JavaScript in a straightforward project, not nearly as complex or difficult as V8.

[1] https://bellard.org/quickjs/

hackthemack

3 months ago

1 reply

QuickJS is amazing. You can put in javascript code, run it through QuickJS and make little binary utilities to run on their own.

Someone took QuickJS and put it in wasm so you can run QuickJS in the browser or in node.

https://github.com/justjake/quickjs-emscripten

Fabrice Bellard is on another planet when it comes to programming. He also wrote FFmpeg and QEMU (among other things).

baudaux

3 months ago

I put QuickJS in https://exaequos.com. You can do graphics app with raylib

jhgb

3 months ago

1 reply

To be fair, there's no claim being made that this was supposed to be a from-scratch implementation of Javascript. Just an equivalent to Deno/Node which don't have their own implementation of Javascript either.

ranger_danger

3 months ago

> there's no claim being made that this was supposed to be a from-scratch implementation of Javascript

That is exactly how I interpreted the title of the article.

thdhhghgbhy

3 months ago

1 reply

He's abandoned it now though.

ivankra

3 months ago

1 reply

Not anymore, https://github.com/bellard/quickjs

rurban

3 months ago

Finally he upped his maintainance hat. So we can archive our own github versions from the tarballs, with various patches.

djmips

3 months ago

1 reply

This could useful to reference for when you want to put a JavaScript interpreter in your own custom software. For example I've seen JavaScript used for console game UI systems.

01HNNWZ0MV43FF

3 months ago

Duktape is also good for that I hear https://duktape.org/

measurablefunc

3 months ago

1 reply

How about a formally verified runtime that takes the JS spec & constructs a runtime by converting the spec w/ incremental & verifiable transformations into an executable runtime?

aw1621107

3 months ago

1 reply

I think you'd need a "proper" formal spec (e.g., JSCert's version of ECMAScript 5 in Coq/Rocq [0]) for that to be feasible. Not exactly sure how "verifiable" would work otherwise.

[0]: https://github.com/jscert/jscert

measurablefunc

3 months ago

1 reply

They also use an unverified parser but good to know this exists.

aw1621107

3 months ago

That's true, though I think the general point stands - you need a "proper" formal spec to even begin thinking about a formally verified runtime. Presumably if you have a full formal spec a verified parser should be within reach.

WalterBright

3 months ago

I wrote a JavaScript engine from scratch using C++ back in 2000.

https://www.digitalmars.com/dscript/cppscript.html

I later translated it to D:

https://github.com/DigitalMars/DMDScript/tree/master/engine/...

Github: https://github.com/DigitalMars/DscriptCPP

bryanrasmussen

3 months ago

I guess I would like to see defining your global object in a real use case and adding some functions to your global object that make sense, which admittedly once you ask someone to do the creative work of making a use case that is sensible as they start implementing it they might find it is more useful to complete the implementation of that use case rather than releasing a starter tutorial.

ramon156

3 months ago

The anchor links seem to be broken

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